How Bird Migration First Evolved: Songbirds Fled the Cold in the North

Songbirds migrate for thousands of miles each year between locations where they breed and raise their young and the areas where they spend their winter. But how did this migration first begin, and how did it evolve? That's what scientists have long wondered and now, researchers may have an answer.

There are two theories when it comes to how migration first began in migratory birds. The first is that ancestors of migratory birds spent the whole year in North America and evolved migration by moving their winter range to the tropics. The second theory is that these bird ancestors were traditionally found in the tropics and evolved migration by moving breeding grounds to more temperate locales.

In order to find out which theory was correct, the scientists created a model to infer how the breeding and winter ranges of migratory species changed through time. Then, they applied this model to a large group of migratory birds that include warblers, cardinals, sparrows, orioles and others. The scientists nicknamed the model itself the "domino model," because the breeding and winter ranges of species were coded in 3x2 grids of binary values, like dots on domino pieces.

The real challenge was reconstructing the most probable evolutionary shifts from one domino to another. Eventually, they found that more evidence supported the idea that birds once lived year-round in North America and eventually began migrating further and further south, resulting in today's migrations. This also suggests that many tropical species of birds are descendants of migratory ancestors that lost migration and stayed in the tropics.

"This is an interesting result because species diversity in this group is much higher in the tropics," said Rick Ree, one of the researchers, in a news release. "Previously, more species in the tropics led to the assumption that temperate, migratory species are derived from tropical, nonmigratory ancestors; however, the results of our phylogenetic study suggest that the opposite pattern happened often in this group."

The findings are published in the journal Proceedings of the National Academy of Sciences.